1. Technical Field of the Invention
The present invention relates to inter-exchange handoff and, in particular, to network operations supporting anchor mobile switching center retrieval of information from a serving mobile switching center following completion of such an inter-exchange handoff.
2. Description of Related Art
Reference is now made to FIG. 1 wherein there is shown a cell diagram illustrating an exemplary cell site configuration for a cellular communications network 100 in which the present invention may be implemented. The cellular communications network 100 operates in accordance with one of a number of known air interface types including, for example, a digital time division multiple access (TDMA) protocol. In a digital TDMA cellular telephone network, for example, each cell site (comprising either an omnidirectional cell, as shown for ease of illustration, or a sector cell) operates with an assigned set of transmission frequencies selected from one or more of the available cellular communications authorized hyperbands (e.g., 800 MHZ, 1900 MHZ, and the like) and frequency bands (A, B, and the like) therein. The set of frequencies assigned to each cell 112 includes frequencies supporting both at least one control channel and a plurality of traffic channels, with the control and traffic channels operable in either or both an analog and/or a digital mode. Sets of assigned frequencies are different for adjacent cells 112, and such sets are not repeated for use by other cells except for those cells that are far enough away from each other to minimize the likelihood of adjacent or co-channel interference.
In the network 100, a base station 114 is provided for each of the cells 112. The base stations 114 engage in simultaneous communications with plural mobile stations 116 operating roughly within the area of the associated cell 112. The control channel assigned to each cell 112 is used to carry system control signals between the base station 114 and proximately located mobile stations 116, and also to assist in the network with mobile station cell reselection. Such control signals include call originations, page signals, page response signals, location registration signals, traffic channel assignments, maintenance instructions, and cell selection or reselection instructions. The traffic channels provided in each cell 112 are used to carry subscriber voice or data communications between the base station 114 and proximately located mobile stations 116 and also to assist in the handoff operation.
The base stations 114 are illustrated as being positioned at or near the center of each of the cells 112. However, depending on geography and other known factors, the base stations 114 may instead be located at or near the periphery of, or otherwise away from the centers of, each of the cells. Each one of the base stations 114 includes a transmitter, a receiver, and a base station controller (none shown) connected to one or more directive antennae (also not shown) in a manner and with a configuration well known in the art.
The base stations 114 further communicate via signaling links and voice trunks 122 with a central control station, commonly referred to as a mobile switching center 118, which functions to control operation of the network 100. A boundary 134 is shown in bold in FIG. 1 to differentiate between those cells 112 (collected in area 132(1)) serviced by a first mobile switching center 118(1), and those cells (collected in area 132(2)) serviced by a second mobile switching center 118(2). The mobile switching centers 118 are interconnected with each other and to the public switched telephone network (PSTN) 120 by signaling links and voice trunks 124. The mobile switching centers 118 operate to selectively connect subscriber voice and data communications to the mobile stations 116 through its base stations 114. Thus, the mobile switching center 118 controls system operation through and in response to the transmission of signals over the control channels to set-up on the traffic channels calls that are either originated by or terminated at the mobile stations 116.
As the mobile stations 116 move within the service area of the network 100, instances arise where a mobile station passes between two cells 112 within a single area 132, or from one cell in a first area 132(1) to another cell in a second area 132(2). In moving between the cells 112, the mobile stations 116, in conjunction with base station 114 collected information and also orders exchanged with and between the mobile switching centers 118, have an opportunity through hand-off to change the base station through which cellular radio communications are being effectuated. For example, a mobile station 116(1) is shown moving in the direction of arrow 126(1) between two cells 112 from area 132(1) into area 132(2). Here, an inter-exchange hand-off must occur in order to continue providing call service to the mobile station 116(1). The mobile switching center 118 controls, through and in response to traffic channel signals, the hand-off of a subscriber communication from a traffic channel of one cell 112 to a traffic channel of another cell as the subscriber mobile station 116 roams throughout the cellular service area during an ongoing communication.
Reference is now additionally made to FIGS. 2A-2B wherein there is shown a signal flow and network operation diagram illustrating network operation in connection with a scenario wherein an inter-exchange hand-off is made from a cell 112(1) within a first area 132(1) generally towards cell 112(2) within a second area 132(2). The mobile station 116(1), operating if capable in accordance with known mobile assisted hand-off (MAHO) principles, periodically makes downlink signal strength measurements 202 on the traffic channel (of cell 112(1)) that is currently being used, and also periodically makes downlink signal strength measurements 204 on the control (i.e., measurement) channels of network identified cells 112, including cells 112(2) and 112(3), which neighbor the cell 112(1). These signal strength measurements are reported 206 to the base station 114(1) for the currently serving cell 112(1). The base station 114(1) concurrently makes uplink signal strength measurements 208 on the traffic channel that is currently being used by the mobile station 116(1).
The base station 114(1) processes the mobile station 116(1) reported 206 downlink signal strength measurements (202 and 204), if available, and the base station made uplink signal strength measurements (208) to determine first whether a hand-off is necessary (action 210) and second, if yes, to which candidate cells the hand-off could and/or should preferably occur (action 212). In this example, it is assumed that the base station 114(1) determines 210 from deteriorating measured uplink and/or downlink signal strengths that a hand-off is necessary. It is further assumed that an identification 212 is made of a plurality of candidate cells 112 for hand-off. These candidate cells 112 may include cells in the same first area 132(1) as the current cell 112(1) such as cell 112(3), as well as cells in other areas 132, such as cell 112(2) in second area 132(2). It will, of course, be understood that the decision to hand-off may instead be made by the mobile station 116(1) itself. A request 214 for hand-off including information comprising an identification of the currently serving cell 112(1), the traffic channel being used for communication with mobile station 116(1) in cell 112(1), the time slot (for a digital traffic channel) carrying the cellular communication, the digital voice color code (DVCC), and the list of potential candidate cells 112 for hand-off, is then sent by the base station 114(1) to the serving mobile switching center 118(1), comprising the serving exchange.
With respect to inter-exchange signaling and a potential for inter-exchange hand-off, the currently serving mobile switching center 118(1) signals 218 the cooperating mobile switching center 118(2), comprising the target exchange, requesting verification of connected base station 114 communications capability with the mobile station 116(1) (i.e., a hand-off measurement request). The signal 218, like the request 214 sent by the base station 114(1), includes information comprising an identification of the currently serving cell 112(1), the traffic channel being used for communication with mobile station 116(1) in cell 112(1), the time slot (for a digital traffic channel) carrying the cellular communication, and the digital voice color code (DVCC).
Responsive to receipt of the signal 218, the cooperating (target) mobile switching center 118, such as mobile switching center 118(2), determines in action 220 from the identification of the cell 112(1), which of its served cells, such as cell 112(2), are neighbors (i.e., candidate cells) for hand-off. As an alternative, this neighbor/candidate list may be provided by the currently serving mobile switching center 118(1). The cooperating mobile switching center 118(2) then signals 222 the base station 114, such as base station 114(2), for each of its connected candidate cells, such as cell 112(2), to make a verifying signal strength measurement (action 224) on the traffic channel currently being used by the mobile station 116(1) in the currently serving cell 112(1).
Additionally, at or about the same time, the mobile switching center 118(1) has similarly identified in action 220 which of its connected cells 112 are neighbors (i.e., candidate cells) to cell 112(1) for hand-off. The mobile switching center 118(1) then similarly signals 222 the base station 114 for each of its connected candidate cells 112, such as the cell 112(3), to make a verifying signal strength measurement (action 224) on the traffic channel currently being used by the mobile station 116(1) in the currently serving cell 112(1).
Each base station 114 instructed by a received signal 222 then reports 228 the results of the verification signal strength measurement to their serving mobile switching center 118 (in, for example, a hand-off measurement report). In the case of reports 228 made to cooperating mobile switching centers 118, such as mobile switching center 118(2), the reported verification signal strength measurements are forwarded 230 on to the mobile switching center 118(1). The verification signal strength measurement results are then processed (action 232) by the mobile switching center 118(1) to determine which one of the candidate cells 112 comprises the best (i.e., the target) cell for hand-off of the call 200 based on the success and strength of the verification signal strength measurement.
Assuming now that the identified target cell for hand-off comprises the cell 112(2) in the area 132(2) , the serving mobile switching center 118(1) requests from the target mobile switching center 118(2) assignment and reservation of a traffic channel (and time slot therein for a digital traffic channel) for hand-off of the call 200 in signal 234. The base station 114(2) and mobile switching center 118(1) are then informed 236 of the assignment by the mobile switching center 118(2) of the traffic channel in the target cell 112(2). The mobile switching center 118(1) then signals 240 the mobile station 116(1) via the base station 114(1) for the currently serving cell 112(1) with a handover command directing the mobile station to switch to the assigned traffic channel (and time slot therein if appropriate) in the target cell 112(2). The mobile station 116(1) then tunes to and accesses 242 the assigned traffic channel (in the proper time slot). When the base station 114(2) detects the mobile station access (action 244), the mobile switching center 118(1), now comprising the anchor exchange, is informed 246, and the call 200 is switched 248 to the mobile switching center 118(2), now comprising the serving exchange, for further handling to complete the hand-off procedure.
It should be noted that the anchor exchange remains connected in handling the call following the inter-exchange handoff using the trunk connection 124. It should further be noted that additional inter-exchange handoffs subsequently may be performed (with the anchor exchange still remaining connected in handling the call). If path minimization techniques are implemented, the target exchange is substituted for the currently serving exchange by the inter-exchange handoff process. In the event no path minimization is implemented, the currently serving exchange becomes a tandem exchange (with the anchor exchange and tandem exchange still remaining connected in handling the call), and the target exchange becomes the new serving exchange.
Reference is now made to FIG. 3 wherein there is shown a message flow and network operation diagram illustrating operation of the network of FIG. 1 following an inter-exchange hand-off of FIGS. 2A-2B to push information from the anchor exchange down to the serving exchange. Although not illustrated, it will be understood that the process for pushing information from the anchor exchange to the serving exchange may implicate and pass through one or more tandem exchanges in cases where path minimization techniques are not implemented. In certain situations where it becomes necessary for the anchor exchange 300 to pass information on to the serving exchange 302 following an inter-exchange handoff, the anchor exchange may originate an ANSI-41 information forward message (INFOFWD) 304 for delivery to the serving exchange (see, TIA/EIA-41.3-D, Section 4.12). Parameters within the information forward message 304 contain the information at issue that must be delivered to the serving exchange 302. For example, changes in the message waiting status with respect to the currently being served mobile station 306 may need to be pushed down to the serving exchange, and the information forward message 304 provides the mechanism for information delivery. Other kinds of information that may be pushed down include: type of announcement/tones to play; message waiting information; and, CNI digits in ASCII format (representing a calling number or redirecting number). Responsive to receipt of the information forward message 304, the serving exchange 302 acts on the included information as is appropriate (action 308) and originates an information forward return result message (infofwd) 310 acknowledging receipt of the message and the taking of the appropriate action.
In the context of a completed inter-exchange handoff of a mobile station communication, an anchor exchange makes a request for certain information from a currently serving exchange relating to the currently being handled cellular call. Responsive to the request, the serving exchange obtains the requested call related information and issues a reply back to the anchor exchange including that information. The inter-exchange call related information request may comprise a suitably formulated ANSI-41 message whose included parameters identify the call related information that is being requested. The included parameters of a corresponding ANSI-41 return result message then provide the requested call related information to the anchor exchange.